The present disclosure is directed to a motor driven, high velocity centrifuge, and more particularly to a centrifuge which incorporates a horizontal rotating drum which is capable of separating particulate material or solids from a liquid. The apparatus finds use in separating oil and water mixed in an emulsion or particles in a liquid such as liquids formed in vegetable and meat processing in food plants, slaughter houses and the like.
The apparatus is directed to a horizontal shaft centrifuge having a screw with flights on the shaft exterior and arranged on the interior of a rotating bowl. The preferred form is stainless steel which resist the corrosion encountered in many types of fluids. At one end, a-slurry is introduced. The slurry is formed of liquid and solid particles, or alternately is formed of immiscible liquids which have differing densities. A relatively large motor, typically 50, perhaps 100 or more horsepower in size, is used to rotate a bowl which drives a screw conveyor aligned axially, in the bowl, where the bowl rotation is imparted to a planetary gearbox connected from the bowl to the screw conveyor. One end of the screw connects to the gear to transfers rotative movement from the bowl to the screw. Both the bowl and the screw rotate together and rotate at almost the same speed. They are not synchronized because there is a difference in speed. As an example, assume for purposes of discussion that the bowl is rotated at 4000 rpm. Assume further that the screw conveyor rotates almost at that speed, this difference being accomplished by the gear box. The difference in the two speeds relates to the recovery of the separated solids. Very briefly, this scrolling rate, represented by the difference in the two speeds (known as the scrolling rate), is preferably reduced to obtain a longer residence time so that the solids are more readily recovered from the centrifuge. Resident time enables settling of the smallest of solid particles as they are collected and as they migrate towards the solid outlet of the bowl. It is desirable on the one hand to extend the residence time to recover more of the solids. On the other hand, it is desirable to reduce the residence so that the throughput of the system is increased. In seeking a balance in the residence time, adjustments must be made periodically to the scroll rate.
In an example, in assuming 4000 rpm for the bowl, if the scrolling rate is 6 rpm, this means that the screw has to rotate at 3994 rpm. A transmission interconnects the rotating members so that this small differential in speed is obtained. Nevertheless, the present apparatus provides a scrolling control mechanism useful with a gear box and back drive in a centrifuge which enables control of the scrolling rate in the range of 50 rpm and less. Scrolling rate is adjusted as will be detailed.
Presently, there is a gear box connected between the bowl of a centrifuge and the screw on the interior. The purpose normally is to use the gear box so that a fixed scrolling rate is achieved between the rotating screw and the surrounding drum. In this instance, the present invention incorporates a separate motor. This motor is provided with a frequency controlled, constant torque drive system. More particularly, torque output is measured. As the torque requirements change, a change in frequency is implemented. The change in frequency coupled with a drive amplifier enables a motor to be driven in synchronous fashion with the frequency source. This causes the equipment to operate at a controllable scrolling rate. The variable frequency drive motor is input through the gear box so that the gear box changes the transfer of power coupled from the main drive motor through the bowl. This will change the relative rotational speed of the bowl and screw, thereby changing the scrolling rate.
The present apparatus is summarized as a system involving a separate motor connected with the gear box for a horizontal centrifuge. More specifically, the centrifuge includes a central screw mounted on a shaft with a bowl rotated by a large motor. The large motor provides adequate power for rotation of the bowl and additionally couples rotative torque through the gear box so that rotation is imparted to the screw conveyor which is mounted for rotation in the bowl. A mix or slurry is introduced at a central location along the screw and the mixture of solids and liquids are centrifugally forced against the bowl. Because of the scrolling interaction of the flights on the screw with the surrounding bowl, scrolling a separating force is imparted to the particles making up the slurry. As a result of the interaction of the screw flights with the bowl, the solid particles are conveyed toward the conical end of the bowl and the liquid is displaced toward the opposite end of the bowl. This separation of liquid from solids results from differences in particle density and also because there is a relative difference in velocity between the screw and the bowl accomplished through a connective gear box. Additional slurry is added at a controlled rate to provide a flow from the two bowl outputs, one being the separated solids and the other being the liquids. The present invention, fortunately, provides an additional power input to the gear box. In conjunction with a frequency meter, variable frequency oscillator, power amplifier, and frequency responsive motor, it is possible to change the backdrive velocity so that the gear box makes the screw and bowl rotate at almost the same speed resulting in nearly synchronous rotation. The scrolling rate is controlled dependent on torque. This enables the scroll rate to be controllably determined, thereby obtaining the desired rate of rotation and thereby obtaining increased or decreased residence time.